1. Field of the Invention.
The present invention relates to a novel method for detecting and identifying magnetic materials through their inherent magnetic characteristics or signatures, and more particularly, to a method that utilizes a uniform interrogating electromagnetic field in which said magnetic materials are immersed in a specific position.
2. Description of the Related Art.
Many systems have been developed in the past for the detection of specific magnetic materials when exposed to a zone where an interrogating electromagnetic field exists. These systems are extensively used in the electronic article surveillance (EAS) business. The interrogating electromagnetic field is generated so that it optimizes the detection of magnetic materials or "tags" thereby preventing the removal (shoplifting) of articles from a store. The most important performance characteristics of these systems are their selectivity and sensitivity. The selectivity characteristic requires that only the specific magnetic material or tag for which a system is designed to detect, is such that false alarms are minimized. It should be noted that some materials can fool such a system at least sometimes.
The other characteristic of these systems is their desired sensitivity so that the tag that carries the magnetic material will produce a field of sufficient intensity that it can be detected regardless of the position at which it crosses the aforementioned electromagnetic field. Most systems have spaces where the electromagnetic field is cancelled thereby forming "holes" where the magnetic material is not exposed to the necessary field intensity.
To optimize these characteristics, systems designed in the past have used active and passive tags as well as various types of fields such as low frequency, microwave, infrared and other electromagnetic fields in different areas of the spectrum. Some designs also use a combination of an electromagnetic field that activates a transducer resulting in a mechanical response in the audible range.
In other more popular systems the selectivity is based on the Fourier analysis of the responding field generated by the specific tag material when it enters the interrogating electromagnetic field. In these systems, the fundamental frequency used to generate the interrogating electromagnetic field activates the tag material which in turn produces a responding field. The responding field so generated is received through an antenna and then processed for the identification of the tag based on a Fourier analysis of the received signal. In this manner, the amplitude and/or phase of one or more of the harmonics of the fundamental electromagnetic field are obtained through the use of a variety of band pass filters, phase comparators, integrator circuits and voltage detectors. When the correct combination of amplitude and/or phase of one or several harmonics matches the corresponding field then a particular "signature" of a given magnetic element is identified.